1) Field of the Invention
The present invention relates to a process for manufacturing hydroxyalkyl ester monomers of a dicarboxylic acid which are well-known by those skilled in the art to be precursors for polyester. In general, the present invention is directed to a continuous pressure process for ethoxylating an aromatic dicarboxylic acid, e.g. terephthalic acid (TA), with an alkylene oxide, e.g., ethylene oxide (EO), without the generation of water in the reaction to form the hydroxyalkylester monomers.
2) Prior Art
It is known in prior art synthesis of hydroxyalkyl ester monomers, such as terephthalate ester monomers (comprising bishydroxyethyl terephthalate, BHET, and/or monohydroxyethyl terephthalate, MHET) that the difficulty in formation of such esters is their limited solubility in most solvents usable in commercial practice, including water. Therefore in a heterogeneous process system in which the catalyst is in the liquid phase, a substantial rate limiting step is the migration of TA into the surrounding liquid phase so that there is sufficient contact area between the TA, EO system and the catalyst.
Another important requirement for the manufacture of these specific terephthalate ester monomers relates to the prevention of further additions of ethylene oxide or glycol to form the alkylether bonded oxygen by-products (including diethylene glycol and triethylene glycol). Eliminating the occurrence of or maintaining a low percentage yield of these by-products is required to minimize their incorporation into the polymer chain, resulting in acceptable polyethylene terephthalate (PET) resin properties such as chemical stability, color stability, thermal stability, crystallization and dyeability properties.
There have been two approaches to the production of BHET for PET from EO and TA based on whether one begins with highly purified TA, or with lower purity TA. If lower purity TA is used, a purification step for the BHET product is required. By-products produced remain with the monomer unless this purification step is undertaken. Little attention has been given to developing a viable commercial route from lower purity TA.
Ethoxylation of high purity TA without isolation or purification of the esterification product is disclosed in U.S. Pat. No. 3,520,853 to Munakata et al. Therein, an amine catalyzed, non-aqueous ethoxylation of TA is disclosed which is conducted in an organic solvent at from 80.degree. C. to 130.degree. C. The Munakata et al. process is conducted at a mole ratio of EO:TA of 2:1 and higher, such that conversion of TA is predominantly to BHET. Removal of EO is conducted at atmospheric pressure at a temperature no higher than 180.degree. C. until the residual EO is below about 0.06 mole per mole of the BHET or less according to a specified equation. The preferred amount of solvent used is 20% to 200% by weight based on the weight of the TA, and it is taught that there is reduced formation of by-product at this solvent level so long as the residual EO is brought below the specified level before the temperature is raised above 180.degree. C. for the polycondensation step.
A process for producing a monomer having a specified carboxyl:hydroxyl end group ratio is described in U.S. Pat. No. 3,551,386 to Berkau. Berkau found that when reacting dicarboxylic acids with glycols (an esterification reaction) under any suitable reaction conditions, the critical objective is to obtain a prepolymer having a CEG:HEG ratio of from 0.05 to 0.46. A monomer having this ratio range undergoes polymerization at a faster rate, however with a CEG:HEG ratio of above 0.46, the polymerization rate drops to near zero because under the conventional conditions of vacuum polycondensation, there are insufficient hydroxyl end groups to increase the degree of polymerization (DP). A faster reacting monomer would be of industrial importance.
An aqueous ethoxylation process to produce BHBT is disclosed in U.S. Pat. No. 3,052,711 to Glogau et al., using a pipe reactor, wherein a salt of TA is reacted continuously with EO with a short exposure to high temperature and the product is cooled immediately thereafter to avoid a high incidence of by-product formation. From about 1.8 to 2.8 moles of EO per mole of TA is fed continuously to the reaction zone with from about 6 to 20 moles of water and a water soluble base material. The predominant esterification product obtained is BHET with a small amount of MHET, at about a 10:1 mole ratio of BHET:MHET.
On the basis of the repeating unit for polyethylene terephthalate: ##STR1##
the mole ratio of EG (ethylene glycol) and TA is one (1). The mole ratio of the starting materials EG and TA in BHET which has the following structure: ##STR2##
is two (2) whereas the mole ratio of the starting materials in MHET, which has the following structure: ##STR3##
is one (1). The BHET and MHET ester monomers can be obtained by reacting either EG or EO with TA. Water is evolved when EG is used while no water is generated when EO is employed. In the present invention, `overall mole ratio` refers to the mole ratio of glycol or glycol precursors (for example: EO) to dicarboxylic acid. Specifically, in a mixture of TA, MHET and BHET, an overall mole ratio of EG to TA is calculable from the mole percent of each component contained in the mixture. Thus, a mixture consisting of TA, MHET and BHET which has an overall mole ratio of EG to TA of 1.2 and which contains 30 mole % unreacted TA, cannot contain more than 50 mole % BHET. Note that for the calculation of mole ratio, both reacted and unreacted TA are included. To clarify, we illustrate the calculation for this mixture below.
Moles in Product Equivalent Component (basis 100 moles total) Moles EG Equivalent Moles TA MHET 20 20 20 BHET 50 100 50 TA 30 0 30 Total 120 100 EG:TA mole ratio 1.2